1. Field of the Invention
The present invention relates generally to a method and an apparatus for increasing energy efficiency in a wireless sensor network, and more particularly to a method and an apparatus for transmitting sensor data using AER and collaborative sensing.
2. Description of the Related Art
Various developments in communication technology have resulted in an environment in which users can freely access a network regardless of location and without taking a computer or network into consideration. This is commonly referred to as a “ubiquitous” environment.
The core technology of a ubiquitous environment includes a wireless sensor network system. In a wireless sensor network system, a sensor node independently operates by detecting information including geographical, environmental, and social changes. The detected information data is transmitted to a sink node of the sensor network. In the wireless sensor network system, each sensor node performs a routing function for transmitting sensor data. Each sensor node performs a routing function because the energy consumption rule in a wireless communication requires consumption of energy two to four times as much as that of the distance that a signal is to be transmitted. Accordingly, when a sensor node is not adjacent to a sink node in the network route, rather than transmit the sensor data directly to the sink node, the sensor node can reduce energy consumption by indirectly transmitting sensor data to the sink node via an adjacent node or through multi-hop.
The basic structure of a wireless sensor network includes multiple sensor nodes, each having an independent sensing capability and a computing capability, which are interconnected through a communication network, wherein each node has its own local battery for supplying power to itself. However, the battery supplying the power to each node has a limited power in comparison with general commercial electricity and is thus very limited in the use of energy.
In order to overcome this problem, research has been conducted for reducing power consumption in all fields of the sensor network. The main stream of this research is to reduce the times of wireless communication or the quantity of wireless communication between nodes, in order to extend the survival time of the network.
FIG. 1A schematically illustrates a typical wireless network system in which a sensor node transmits data to a sink node and the sink node performs a sensor data operation. In contrast, FIG. 1B schematically illustrates a wireless network system improved for increasing the energy efficiency of a sensor network, in which data aggregation (in-network aggregation) is used for transmitting data from a sensor node to a sink node.
More specifically, the in-network aggregation technology as illustrated in FIG. 1B has been studied as a solution for reducing the existing transmission message communication. As an example of a trial for more efficiently reducing the energy consumption by the communication message, it is possible to apply a threshold that changes according to the location of a node, i.e., to apply a threshold that increases as a node to which data is to be transmitted becomes farther from a node at which the data is to be finally collected. It is possible to reduce the message quantity by preventing transmission of sensor data to a higher node under the threshold. Here, an average value and a standard derivation are obtained from sensor data collected during a predetermined time from a predetermined group of nodes, and the obtained average value and standard derivation are compared with those obtained for a previous time, such that a predetermined threshold is obtained. Thereafter, data is transmitted only when a result of the comparison exceeds a predetermined threshold.
However, the threshold reflects the distance from the sensor node to the sink node, and a long distance increases the number of messages necessary for transmission to the sink node, thereby increasing the energy consumption. Therefore, the threshold causes allocation of less sensor data to a node located far from a sink node than to a node located near the sink node. Although the threshold is determined to have an optimum value either by a user or according to applicability, there is no indication of a specific solution for dynamic control. Further, although the transmission technology described above utilizes a distance factor, which implies that the technology discriminates the sensor data from a distanced node, it is not preferable because occurrences of events at the sensor node have no relation with the distance to the sink node.
Therefore, there is still a need for another solution, which increases energy efficiency of a sensor node by transmitting sensor data based on a category capable of properly selecting sensor data transmitted to a sink node.
The problems of a wireless sensor network system as described above can be summarized into the following two subjects: (1) prolonging the life of a sensor node by reducing energy consumption of the sensor node as much as possible; and (2) achieving economic and efficient use of an entire wireless network system through uniform energy consumption by multiple sensor nodes.